Electron Beam Physical Vapor Deposition Thermal Barrier Coatings Research Articles

A New Al2 O3 -Protected EB-PVD TBC with Superior CMAS Resistance.

Calcium-magnesium-aluminium-silicate (CMAS) attack is a longstanding challenge for yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) particularly at higher engine operating temperature. Here, a novel microstructural design is reported for YSZ TBCs to mitigate CMAS attack. The design is based on a drip coating method that creates a thin layer of nanoporous Al2 O3 around YSZ columnar grains produced by electron beam physical vapor deposition (EB-PVD). The nanoporous Al2 O3 enables fast crystallization of CMAS melt close to the TBC surface, in the inter-columnar gaps, and on the column walls, thereby suppressing CMAS infiltration and preventing further degradation of the TBCs due to CMAS attack. Indentation and three-point beam bending tests indicate that the highly porous Al2 O3 only slightly stiffens the TBC but offers superior resistance against sintering in long-term thermal exposure by reducing the intercolumnar contact. This work offers a new pathway for designing novel TBC architecture with excellent CMAS resistance, strain tolerance, and sintering resistance, which also points out new insight for assembly nanoporous ceramic in traditional ceramic structure for integrated functions.

Thermal shock behavior of EB-PVD thermal barrier coatings based on YSZ ceramic coat and (Ni, Pt)Al metallic bond coat

Three different chemical vapor deposition (CVD) aluminizing processes were adopted for preparing of (Ni, Pt)Al metallic coatings, whose microstructure, element content, phase constituent and gas hot corrosion performance were evaluated and comparatively analyzed. YSZ ceramic coatings were subsequently fabricated via electron beam physical vapor deposition (EB-PVD) technique on top of the optimized (Ni, Pt)Al bond coat surface, and thermal barrier coating (TBC) specimens were subjected to 1373 K long-term thermal shock. The surface of aluminized sample with a single external reaction generator is relatively compact, and no defects such as micropores and microcracks are observed. Only a large number of protruding sediments are detected at grain boundary as well as the degree of rumpling and cross-links of these sediments is more obvious. A large amount of Pt is enriched at that grain boundary, while elemental content of Al is lower at the same location. Meanwhile, such sample basically displays β-(Ni, Pt)Al phase, and only three very weak diffraction peaks belonging to ζ-PtAl2 coexist. After long-term thermal shock, the exposed region of bond coat shows two kinds of micro-morphology. The biggest distinction between them is that the composition of Pt and Al elements is evidently different. The interfacial separation of TBCs is mainly concentrated at interface between thermally grown oxide (TGO) and bonding layers, and a very small amount of TGO layer is merely adhered to bond coat surface. It demonstrates that TGO layer is densified and thickened with the extension of thermal exposure period, and then the mismatching of thermal expansion between Al2O3 and (Ni, Pt)Al bond coat occupies the dominant factor. In addition, the grain boundary ridges and undulations on bonding layer surface can also induce accumulation of the tensile stress and further accelerate degradation of the critical interlayer interface during cooling stage of thermal shock.

High-Temperature Solid Particle Erosion of Environmental and Thermal Barrier Coatings

Solid particle erosion (SPE) is a common phenomenon observed in gas turbine engines. Particles entrained in the gas flow impact engine hardware, resulting in micro-scale damage that leads to deleterious effects such as material removal. For protective coatings, damage due to SPE is a key concern, since it can negatively affect the durability of the coating and subsequently the life of the underlying component. In this work, the high-temperature SPE behavior of two state-of-the-art environmental barrier coatings (EBCs) deposited via air plasma spray (APS) is investigated using alumina erodent to understand the effect of particle kinetic energy, impingement angle, and temperature. The SPE behavior of the EBCs is also compared to APS and electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) to elucidate similarities and differences in the erosion response. The EBCs were more susceptible to SPE than the EB-PVD TBC but had greater SPE resistance compared to the APS TBC. Coating microstructure and porosity were shown to have a strong influence on the observed behavior.

Evaluation of Solid Particle Erosion of EB-PVD TBCs under Thermal Cycling Conditions Based on a Stochastic Approach

The solid particle erosion behavior of electron-beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) was numerically evaluated under thermal cycling conditions. The erosion rates were calculated based on the mechanics-based formulae where the model parameters are fitted to the temperature-process-dependent test data available in the literature. A stochastic approach was applied to simulate the erosion behavior toward service conditions. The mechanics-based formulae were then validated by experimentally measured temperature and sintering-dependent erosion rates from the literature. The pseudoductile erosion behavior is identified for silica particles in the EB-PVD topcoat (TC) erosion system above the intermediate temperatures (~220 °C) due to the softening of partial molten silica particles, thus leading to an increase in the cutting wear and a decrease in deformation wear. The erosion rates are found to decrease versus temperature but increase versus thermal cycles. Such erosion behavior could be attributed to propagation of sintering cracks induced at elevated temperatures. The parametric calculations show that both erosion and thermal cycling parameters have a profound effect on the erosion mechanism of EB-PVD TC. The erosion rate increases at higher solid particle velocity and accumulated mass but displays a pseudoductile erosion behavior versus variation of impacting angles. Two types of erosion mechanisms were evaluated under different thermal cycling conditions. Under the burner cycling test with a short high-temperature dwell period, the erosion mechanism of EB-PVD TBCs is governed by temperature, while under an isothermal cycling test with a high-temperature long dwell period, the erosion is determined by sintering time. The failure mechanisms of EB-PVD TBCs under solid particle erosion processes are discussed combining internal cracking within topcoat and external erosion on the surface of topcoat.

Thermally Grown Oxide Stress in PS-PVD and EB-PVD Thermal Barrier Coatings Observed at Various Lifetimes Via Synchrotron X-ray Diffraction

Abstract The current standard application method for thermal barrier coatings (TBCs) on turbine blades for jet engines is electron-beam physical vapor deposition (EB-PVD) due to its high strain tolerance and low thermal conductivity. An emerging deposition method, plasma-spray physical vapor deposition (PS-PVD), presents an opportunity for a tailorable microstructure, and non-line-of-sight deposition that is faster and less expensive. To compare the lifetime behavior of both PS-PVD and EB-PVD coatings, samples subjected to 300 and 600 thermal cycles were measured during a 1 h thermal cycle to determine the strains, which were converted to stress, in the thermally grown oxide (TGO) layer of the TBCs using synchrotron X-ray diffraction (XRD). Room temperature XRD measurements indicated among samples that PS-PVD coatings experienced greater variation in in-plane room temperature strain in the TGO after cycling than the EB-PVD coatings. In-situ XRD measurements indicated similar high-temperature strain and no spallation after 600 thermal cycles for both coatings. Microscopy imaging after cycling showed greater rumpling in PS-PVD coatings that led to different failure modes between the two coatings’ TGO layers. The tailorability of PS-PVD coatings allows for adjustments in the processing parameters to improve their overall performance after aging and bridge the differences between the two deposition methods.

Failure Mechanism of EB-PVD Thermal Barrier Coatings under the Synergistic Effect of Thermal Shock and CMAS Corrosion

Thermal barrier coatings (TBCs) suffer from the thermo-chemo-mechanical coupling action of thermal shock and calcium–magnesium–alumina–silicate (CMAS) corrosion. However, the failure mechanism of TBCs under the synergistic effect of thermal shock and CMAS corrosion is still unclear due to a lack of an environmental simulator. Herein, an 8YSZ ceramic coating is deposited on a PtAl bond coating/DD419 nickel-based single crystal superalloy substrate using the electron beam physical vapor deposition (EB-PVD) method. The thermo-chemo-mechanical coupling effect of TBCs is achieved in a self-developed environmental simulator. The interaction of volume expansion induced by the phase transition of ZrO2, structural degradation and thermal fatigue further increases the out-of-plane tensile stress and in-plane shear stress in the ceramic coating, which accelerates the initiation and propagation of surface vertical cracks and horizontal cracks. As multiple surface vertical cracks propagate to the interface and merge with interfacial cracks, the ceramic coating spalls from the substrate.

The Effect of Bond Coat Roughness on the CMAS Hot Corrosion Resistance of EB-PVD Thermal Barrier Coatings

In a high-temperature, high-flame-velocity, and high-pressure gas corrosion environment, the intercolumnar pores and gaps of electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) may serve as infiltration channels for molten calcium–magnesium–alumino–silicate (CMAS), leading to the severe degradation of TBCs. In order to clarify the relationship between the roughness of the bond coat and the CMAS corrosion resistance of the EB-PVD TBCs, 7 wt.% yttria-stabilized zirconia (7YSZ) TBCs were prepared on the surfaces of four different roughness-treated bond coats. The effect of the bond coat roughness on the columnar microstructure of the EB-PVD YSZ was investigated. The effect of the change of the bond coat’s microstructure on the CMAS corrosion resistance of the EB-PVD YSZ was studied in detail. The results showed that the reduction in the roughness of the bond coat contributes to the improved formation of the EB-PVD YSZ columns. The small and dense columns are similar to a lotus leaf-like structure, which could reduce the wettability of CMAS and minimize the spread area between the coating and the CMAS melt. Thus, the CMAS corrosion resistance of the coating can be greatly improved. This preparation process also provides a reference for the preparation of other TBC materials, improving the resistance to CMAS hot corrosion.

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with T max = 1100 °C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged. TBC thickness had only a limited influence on cyclic lifetime.

Influence of particle shape on erosion behavior of EB-PVD thermal barrier coatings

In a thermal barrier coating (TBC) system with a complicated structure and a demanding service environment, erosion is a common failure mode. The influence of particle shape on erosion failure of electron beam physical vapor deposition (EB-PVD) TBCs was investigated using the finite element method in this study. There were two types of particles considered: ellipsoidal and spherical. To describe the particle shape, the geometric parameter long-to-short axis ratio λ was introduced. There were also two cases for ellipsoidal particles: long-axis erosion and short-axis erosion. The effect of particle velocity and erosion angle was taken into account. The results show that the shape of the erosion particle affects the maximum erosion depth, rebound velocity, and maximum residual stress for the same initial kinetic energy. In general, the ceramic layer erosion depth, rebound velocity, and interfacial stress S22 for ellipsoidal short-axis erosion are close to those for spherical erosion. The maximum erosion depth of the ceramic layer, rebound velocity, and interfacial residual stress S22 for ellipsoidal long-axis erosion, on the other hand, are significantly larger than those for the other two types of erosion, and these results for the long-short axis ratio of 3 are significantly larger than those for the long-short axis ratios of 2 and 3/2. The theoretical analytical model was used to validate the reliability of the finite element results. The findings indicate that the influence of erosion particle shape must be considered when studying the erosion failure mechanism of EB-PVD TBC.

A case study on CMAS corrosion of an in-situ microstructure modification YSZ TBCs fabricated by EB-PVD

At elevated temperature, the inter-columnar pores of electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) may act as infiltration channels for the molten calcium–magnesium–alumino-silicate (CMAS) and, therefore, makes it susceptible to be attacked by CMAS. To enhance the CMAS corrosion resistance of EB-PVD TBCs, a microstructure modification yttria-stabilized zirconia (YSZ) top coat consisting of a low porosity fibrous structure layer on the conventional columnar structure layer was fabricated by in-situ parameter modification. The anti-CMAS corrosion performance of the in-situ modified YSZ coating was evaluated at 1250 °C and the evolution of the microstructure was investigated in detail. The results show that the inter-columnar pores were sealed by the low-porosity and smooth fibrous structure layer. Due to the porosity and the size of the pores of the fibrous structure layer are dramatically reduced, the penetration rate of CMAS is significantly lower in the in-situ microstructure modification coating than in the conventional columnar structure coating, and the interaction between CMAS and YSZ coating was also reduced.

Failure behavior study of EB-PVD TBCs under salt spray corrosion and thermal shock cycles

Thermal barrier coatings (TBCs) are the most used methods to protect the internal hot end components of aeroengines for their improved thermal efficiency. Because of the severe service environment of TBCs, their spalling failure poses a problem. In recent years, studies on the erosion resistance of TBCs in marine atmosphere have attracted much attention. The study made an initial attempt in building a simulation experiment of aeroengine TBCs under marine atmospheric circumstances to explore the effect of thermal shock on the performance of TBCs under salt spray corrosion. The experimental results show that electron beam-physical vapor deposition (EB-PVD) Y2O3-stabilized ZrO2 (YSZ), one of the mainstream TBCs technologies, produced 20% of the surface spalling area after 300 thermal shock cycles under the simulated experimental conditions of salt spray corrosion and thermal shock. By testing and analyzing the failure samples and combining the results of SEM, EDS, and XRD, it was found that the ceramic layer and thermally grown oxide (TGO) are partly dissolved, thus dispersing coatings’ particles, but the phase structure does not change significantly, indicating the fine anticorrosion property of EB-PVD TBCs.

Rheological and chemical interaction between volcanic ash and thermal barrier coatings

Combustion temperatures (>1200 °C) in gas turbines cause softening and eventually melting of ingested volcanic ash particles. In the downstream section, lower temperatures lead to droplet deposition onto the thermal barrier coatings (TBCs) of the turbine blades. The intensity of interaction (wetting and chemical interaction) depends on the ash and TBC chemistry as well as the TBC structure. We have determined the spreading behavior of five different volcanic ash melts on four types of TBCs. The TBCs differed in their composition - yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) – and in their fabrication - air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). The spreading properties have been parameterized on the basis of four parameters, 1) the CaO-SiO 2 -ratio, 2) optical basicity, 3) the R b/a ratio and 4) the viscosity of the volcanic ash melts. Infiltration efficiency of the melts into the TBCs and corrosion characteristics have been determined via electron microprobe analysis of cross sections. In this study, the highest damage potential was found following interaction with basaltic melt, as its low viscosity is favorable to extensive spreading and high chemical reactivity. • Basaltic (SiO 2 -lean) silicate melts show the most extensive spreading on TBC surfaces and the most intense chemical reaction with the TBC materials • Spreading of silicate melts on APS TBCs is enhanced over EB-PVD TBCs and spreading on GZO TBCs is enhanced over YSZ TBCs • Spreading ability of silicate melts can be estimated based on a parametrization deduced from our spreading experiments

Thermal Analysis of Calcium–Magnesium–Alumino–Silicate Infiltration Dynamics in Thermal Barrier Coatings

Molten calcium–magnesium–alumino–silicate (CMAS) infiltration into thermal barrier coatings (TBCs) of gas turbines causes loss of strain tolerance and delamination of the ceramic topcoat. To develop efficient mitigation strategies, it is crucial to understand CMAS infiltration dynamics into the porous topcoat. This study introduces an integrated model, incorporating liquid flow in unsaturated porous structures, heat transfer, and temperature-dependent viscosities, to study CMAS infiltration through TBCs grown by the electron beam physical vapor deposition (EB-PVD) method. The effects of different CMAS compositions, temperature gradients across the topcoat, and coating microstructures are investigated. Our simulation shows that CMAS infiltration exhibits significantly nonlinear dynamics with a fast infiltration rate at the early stage due to high temperature, high pressure gradients, and low viscosity. Neglecting heat transfer enhancement from CMAS by approximating the temperature distribution as linear underestimates the infiltration rate. Fine porous microstructures slow infiltration, and bilayer or multilayer structures, consisting of variable column and pore sizes, combine the advantages of an increased hydraulic resistance to infiltration and lower capillary pressures. Such heterogeneous structures can delay early-stage infiltration by manipulating the layer thickness and arrangement. It is anticipated that the quantitative information and advanced understanding obtained would benefit the development of CMAS-resistant EB-PVD TBC topcoats.

Thermal Insulation of YSZ and Erbia-Doped Yttria-Stabilised Zirconia EB-PVD Thermal Barrier Coating Systems after CMAS Attack.

The impact of small deposits of calcium–magnesium–aluminium silicates (CMAS) on the top of thermal barrier coatings (TBCs) made of yttria-stabilised zirconia (YSZ) produced via electron-beam physical vapour deposition (EB-PVD) is shown to play a role in the microstructural and chemical stability of the coatings; hence, it also affects the thermal insulation potential of TBCs. Therefore, the present work investigates the degradation potential of minor CMAS deposits (from 0.25 to 5 mg·cm−2) annealed at 1250 °C for 1 h on a novel Er2O3-Y2O3 co-stabilised ZrO2 (ErYSZ) EB-PVD TBC, which is compared to the standard YSZ coating. Due to the higher reactivity of ErYSZ coatings with CMAS, its penetration is limited in comparison with the standard YSZ coatings, hence resulting in a better thermal insulation of the former after ageing.

Effect of processing and interface on the durability of single and bilayer 7YSZ / gadolinium zirconate EB-PVD thermal barrier coatings

Gadolinium Zirconate (GZO) is known for its low thermal conductivity, high thermal stability and a favorable fast reaction with CMAS deposits that makes such a thermal barrier coating (TBC) more stable and less infiltrated in dusty turbine environments. In the present study, Electron Beam Physical Vapor deposited (EB-PVD) GZO/7YSZ bilayers were coated on superalloy bars having a NiCoCrAlY bond coat. Two different coaters that were either pilot production/lab scale size suitable for coating development (coater ESPRI) or small scale production/development size (SMART Coater) suitable for both development and production were used. The bilayers were compared to single layers of 7YSZ or GZO, respectively. Beside lifetime investigations of the various TBC systems special emphasis was put on the interface architecture between 7YSZ and GZO. The coatings where the transition between the two layers was done rapidly achieved the longest lifetime of all bilayers, regardless of the EB-PVD coater used. Nearly all GZO top coats showed a longer lifetime than the standard 7YSZ systems in furnace cycling testing at 1100°C.

Cyclic Hot Corrosion Failure Behaviors of EB-PVD TBC Systems in the Presence of Sulfate and Vanadate Molten Salts

The cold gas dynamic spray (CGDS) method has been considered a promising technology to produce a metallic bond coat for thermal barrier coating (TBC) systems. In this study, CoNiCrAlY bond coats produced by CGDS method were coated with yttria-stabilized zirconia (YSZ) by electron beam physical vapor deposition (EB-PVD). TBCs were subjected to 50 wt % V2O5 and 50 wt % Na2SO4 molten hot corrosion salt combinations at 1000 °C. In the case of YSZ top coat on TBCs, the reaction between Na2SO4, V2O5, and Y2O3 salts generates YVO4 crystals, and these structures cause the transformation of tetragonal ZrO2 to monoclinic ZrO2. This situation occurs under operating conditions that lead to TBC failure. Hot corrosion behavior and the related failure mechanisms of TBC systems were investigated and discussed using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, and X-ray diffractometer (XRD).

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m−1 K−1 for EB-PVD YSZ coatings to about 0.7 W m−1 K−1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni3Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.

Effects of CMAS penetration on the delamination cracks in EB-PVD thermal barrier coatings with curved interface

Abstract During the high-temperature operation of electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC), the penetration of environmental calcium-magnesium-alumina-silicate (CMAS) compositions into the ceramic top-coat would affect the growth of delamination cracks. In this work, the effects of CMAS penetration on the delamination cracks in EB-PVD TBC with curved interface are investigated by finite element analysis. In the numerical model, the curved interface evolves as the cyclic displacement instability of the thermally grown oxide (TGO) layer. The penetration of CMAS into the columnar gaps of EB-PVD TBC mainly increases the in-plane modulus of TC layer. It is demonstrated that, with the increase of in-plane modulus in an intact TC, the level of tensile stress, which mainly occurs in the region above the curved interface and responsible for initiating the delamination cracks, presents a decrease; meanwhile, the level of shear stress, which mainly occurs in the region at the periphery of the curved zone to drive the delamination crack when it propagates into this region, presents a increase. Furthermore, the calculation of the strain energy release rate shows that, for the crack located above the curved interface, the increase of in-plane modulus in TC layer can prevent the accumulation of strain energy release rate, and therefore make it more difficult for delamination initiation. However, once the crack propagates into the flat periphery, CMAS penetration would begin to enhance its growth.

Adherence and Failure of an EBPVD 7YSZ Coating on a β/γ-NiCrAl Substrate: A Pilot Study

Oxidation, lifetime and crack path at failure upon furnace oxidation testing at 1100 °C were evaluated for an undoped alumina-forming NiCrAl alloy/7YSZ (7 wt% yttria stabilized zirconia) coating system. The coating was deposited by electron beam physical vapor deposition (EBPVD). The thermally grown oxide (TGO) was investigated by energy dispersive spectroscopy. The outer zone of the TGO comprised a particular interlayer-denominated mixed zone (MZ), which revealed accumulation of Cr together with Y and Zr. The contents of Y and Zr in the MZ of the undoped sample were in good agreement with compositional data previously evaluated for thermal barrier coating (TBC) systems with EBPVD NiCoCrAlY bond coats versus relative lifetime. These data are considered characteristic measures of the stage of life for EBPVD TBC systems.

Crack prediction in EB-PVD thermal barrier coatings based on the simulation of residual stresses

Thermal barrier coatings systems (TBCs) are widely used in the field of aerospace. The durability and insulating ability of TBCs are highly dependent on the residual stresses of top coatings, thus the investigation of the residual stresses is helpful to understand the failure mechanisms of TBCs. The simulation of residual stresses evolution in electron beam physical vapor deposition (EB-PVD) TBCs is described in this work. The interface morphology of TBCs subjected to cyclic heating and cooling is observed using scanning electron microscope (SEM). An interface model of TBCs is established based on thermal elastic-plastic finite method. Residual stress distributions in TBCs are obtained to reflect the influence of interfacial roughness. Both experimental and simulation results show that it is feasible to predict the crack location by stress analysis, which is crucial to failure prediction.